1. Field of the Invention
This invention relates to phototransistors and more particularly to phototransistors that are useful in the long wavelength infrared region, that is for wavelengths of about 6 to 20 micrometers.
2. Description of Related Art
Heterojunction bipolar phototransistors are well known as devices that can be used to produce an output electrical current in response to the presence of light. See, for example, the article entitled "Gain of a Heterojunction Bipolar Phototransistor" by Naresh Chand et al. in IEEE Transactions on Electron Devices, Vol ED-32, No. 3, March, 1985, pages 622-627. In the Chand et al. article, a phototransistor is described wherein the incident light passes through a transparent emitter and is absorbed in the base, the depletion region between the base and the collector, and the neutral collector. The electron-hole pairs generated in the depletion region and within the diffusion lengths of the minority carriers in the base and collector are separated and collected by the field of the base-collector junction leading to a current flow in an external circuit. The holes are swept into the base and are accumulated near the emitter-base junction due to a large potential barrier in the valence band at the heterointerface until they recombine with injected electrons from the emitter. To maintain the charge-neutrality condition in the base, a large injection of electrons occurs from the emitter into the base resulting in a large electron-current flow from the emitter to the collector. While phototransistors of the type in the Chand et al. article have provided greater gain than a simple photodiode, they have not provided a light detection that is useful in the infrared region of about 6 to 20 micrometers wavelength.
One form of a detector which is useful in the desired range of wavelengths is the quantum well infrared detector wherein p-doped Si.sub.1-x Ge.sub.x layers are grown between undoped Si barrier layers in a quantum well structure in order to form multiple quantum wells primarily in the valence band, The term quantum well structure in this context refers to stacks of ultrathin layers of semiconductor material where the dimension of the ultrathin layer is sufficiently small such that the electron is considered to be essentially confined in that dimension by the energy barriers created by the differences in bandgaps. A description of this type of a photodetector can be found, for example, in the article entitled "Si.sub.1-x Ge.sub.x /Si multiple quantum well infrared detector", by R. P. G. Karunasiri et al., Applied Physics Letters, Vol. 59, No. 20, Nov. 11, 1991, pages 2588-2590. In detectors of the type described in the Karunasiri et al. article, the light excites holes within the quantum well to an extended miniband beyond the quantum well barriers formed by the Si layers, and the transport of holes in the quantum well region provides a current flow in an external circuit. Unfortunately, these quantum well detectors of the prior art have large leakage and large dark current due to the small heterojunction barriers and they therefor must be cooled typically to about 50.degree. K. to reduce the dark current and Johnson noise. In addition these prior art devices have limited photoconductive gain.